Method and apparatus for writing and reading optical recording medium

ABSTRACT

An apparatus for writing on and reading an overwritable optical disk comprises an identifier detector that identifies a recording condition in the sector to be overwritten, and a delay time controller circuit that sets a variation range of the start point for writing according to the recording condition. The record timing of the modulated data signal is changed at random within the set variation range when overwriting the sector of the optical disk.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method and apparatus forwriting on and reading an optical recording medium.

[0002] Recently, optical disks, cards and tapes are developed and havebeen used for recording information optically. Especially, optical disksare given attention as a medium having large capacity and high density.

[0003] A conventional method for writing an optical disk is explainedbelow referring to the figures. FIG. 27 shows an example of an opticaldisk using a phase-change type recording film. A substrate 2301, whichis made of a glass or plastic material such as PMMA or polycarbonate, isprovided with guide grooves 2302 and pits indicating an address or otherinformation. This area with the pit train is called the ID area. Theguide grooves are formed in concentric circles or a coil from the innerto outer portions of the substrate. Areas 2307 between the grooves arecalled lands. The ID areas are located at a predetermined pitch alongthe guide grooves. The areas between the ID areas are called sectors. Asurface of the substrate 2301 is provided with layers of a protectivefilm 2303, a recording film 2304 and a reflection film 2305 formed bysputtering or other methods. Furthermore, a protective sheet is gluedonto the layers.

[0004] A method for writing on and reading the above-mentioned opticalrecording medium is explained below referring to the figures. FIG. 28shows a block diagram of a conventional writing and reading apparatus.FIG. 29 shows the write and read operation for an optical disk. In FIG.29, (a) indicates a write data signal, (b) indicates a laser-drivingsignal (corresponding to a laser power), (c) indicates a recorded stateof the optical disk, and (d) indicates a record format.

[0005] The reading process for the optical disk is performed as follows.A system controller circuit 101 drives a spindle motor 114 that rotatesthe optical disk 113. An optical head 112 focuses a laser beam with aweak power (Pr in FIG. 29) to irradiate the optical disk 113, trackingthe guide groove 2302 and the pit train 2502 shown in (c) of FIG. 29.The intensity of the beam reflected by the optical disk 113 varies inaccordance with the existence of the pit train 2502 and record marks2501. Detecting the intensity of the reflected beam generates readsignal 122, which is processed into binary data by a read signalprocessor circuit 115 and demodulated by a demodulator circuit 116. Thenthe signal is processed in an error correction and deinterleavingcircuit 117 to obtain read data. The deinterleaving process restores theoriginal data from the interleaved data, which are changed in order.

[0006] The writing process for the optical disk is performed as follows.A system controller circuit 101 connected to a host computer gives writedata 102 to an error correction and interleaving circuit 103, which addserror correcting data, i.e., parity bits to the write data, and performsan interleaving process. The interleaving process makes error correctioneasy by converting a burst error (long continuous error) due to a defectof the optical disk into a random error (short error). The write dataare divided into blocks and the order of the blocks is changed accordingto a predetermined rule in the interleaving process. Then a modulatorcircuit 104 modulates the data in accordance with the (1, 7) RLLmodulation method, for example. Consequently, a modulated data signal105 is obtained for writing the data area 604 shown in (d) of FIG. 29.

[0007] In the synthesizer circuit 109, each data block to be writteninto each sector is provided with VFO and RESYNC signals from asynchronizing signal generator circuit 108 as well as dummy data from adummy data generator circuit 107 if necessary, to make the write datasignal 118. The VFO and RESYNC are synchronizing signals for generatinga clock signal synchronizing with the read signal in a PLL circuit(synchronizing signal generator) in the read signal processor circuit115. The VFO signal is added to the head of the modulated data, and theRESYNCH signal is added in the modulated data signal at a predeterminedinterval. The dummy data are added for reducing a deterioration of therecording film generated at the end of writing when writing on the samesector repeatedly. The dummy data is not required to include anyinformation. The example of the write data signal 118 is shown in (a) ofFIG. 29.

[0008] Corresponding to the write data signal 118, the laser drivercircuit 110 generates a laser driving signal 111 to drive a laser in theoptical head 112, modulating the intensity of the laser beam. An exampleof the laser-driving signal 111 is shown in (b) of FIG. 29.

[0009] When the optical head 112 irradiates the recording film of theoptical disk 113 with the focused laser beam having a high intensity (Ppshown in (b) of FIG. 29) for a predetermined period, the temperature ofthe recording film rises above the melting point and drops rapidly. As aresult, the melted spot becomes a recorded mark 2501 (shown in (c) ofFIG. 29) having an amorphous state due to rapid cooling. On thecontrary, when the recording film is irradiated with the focused laserbeam having a middle intensity (Pb shown in (b) of FIG. 29) for apredetermined period, the temperature of the recording film rises to thetemperature below the melting point but above the crystallization point.Then the irradiated spot is cooled gradually and assumes a crystallinestate.

[0010] A recorded pattern having crystalline and amorphous spots asmentioned above, which corresponds to the modulated data signal 105, iscreated in the data area 604 on the guide groove 2302. Thus, writing andreading of information are performed using a difference of reflectivitybetween the crystalline and amorphous states.

[0011] As shown in (d) of FIG. 29, there is a gap area 602 between theID area 601 and the VFO area 603, as well as a buffer area 606 betweenthe dummy data area 605 and the next ID area 601. The gap area 602generates a time for controlling the laser power, and the buffer area606 compensates for a difference of recording position due to rotationvariability of the spindle motor.

[0012] When scanning an ID area 601 between sectors 607 of the opticaldisk, address data are read by the laser irradiating the optical diskwith the same weak power as the reading power.

[0013] The system controller circuit has a configuration shown in FIG.30. Transmission of write data and read data between a host computer andthe write/read apparatus is performed using a write data buffer 2601 andread data buffer 2602 respectively. The read data is given to the readdata buffer 2602 as well as an address data detector circuit 2603. Anaddress data detecting signal is transmitted to the write data buffer2601 and the read data buffer 2602. A motor driver circuit 2604 drivesthe spindle motor.

[0014] When writing on the optical disk repeatedly as mentioned above, aquality of the read signal of the written data in a sector may bedeteriorated at a certain part. Especially, writing similar data intothe same sector repeatedly makes the deterioration serious because thatpart of the sector undergoes repeated melting and hardening whileanother part never melts. As a result, the thickness of the recordingfilm changes at the boundary of the two parts, so that the thermal andoptical characteristics are deteriorated at the boundary. In this case,it is difficult to record (write) and reproduce (read) data properly.

[0015] There is a writing method to solve the above-mentioned problemproposed in the Japanese laid-open patent application (Tokukaihei)2-94113. This method writes data while varying the start point forwriting a sector at random within a predetermined range. This range iscalled the variation range in this specification.

[0016] In this writing method, however, the variation range of the startpoint for writing was constant for various recording media orconditions. On the other hand, the deterioration rate of the recordingfilm depends not only on the number of repeating writings but also onthe recording medium or recording condition.

[0017] Therefore, the above-mentioned writing method in the prior art isnot enough for improving the deterioration of the recording film inevery case. For example, when overwriting the optical disk, the wholesector is overwritten. Therefore, even if the data to be written areonly a small part of the sector, the whole sector is overwrittenactually. A directory area of the disk is overwritten repeatedly withsimilar data. Thus, the directory area has a tendency to have itsrecording film deteriorated earlier than another area (called thegeneral area in this specification).

[0018] Increasing the variation range of the start point for writing mayreduce the deterioration of the recording film. However, an area forwriting VFO or dummy data is decreased in a sector because the data areashould be settled in the sector. In other words, when adding the VFOarea for generating synchronizing data to the head of the data area, andadding the dummy data area to the tail of the data area, the length ofthe VFO area or the dummy data area have to be shortened in accordancewith the enlarged variation range of the start point for writing.Therefore, the deterioration of the recording film at the start and endpoints of the sector may become critical when being written repeatedly,so that reading of the written data becomes difficult at a certaindeterioration level of the recording film. As a result, the number ofoverwriting of the optical disk may be lowered.

SUMMARY OF THE INVENTION

[0019] The present invention provides a method and apparatus for writingon and reading an optical recording medium, which can relieve adeterioration of the recording film properly and increase the number ofoverwritings by changing the variation range of the start point forwriting in accordance with the writing condition.

[0020] A method according to the present invention comprises the stepsof converting write data into a modulated data signal corresponding to arecord pattern on the recording medium, selecting a first or secondwrite timing, altering the start point for writing the modulated datasignal at random within a first variation range in a sector if the firstwrite timing is selected, and altering the start point for writing themodulated data signal at random within a second variation range that islarger than the first variation range in a sector if the second writetiming is selected.

[0021] As mentioned above, the variation range of the start point forwriting is set in accordance with a recording medium or recordingconditions. Thus, deterioration at the specific part of the recordingfilm is relieved when writing repeatedly, and the number of overwritingsis increased by enlarging the variation range in the case of a criticalrecording method or medium. On the other hand, the number ofoverwritings can be increased by lengthening the VFO area or the dummydata area for suppressing the deterioration at the head or tail part ofthe sector in the case of the recording method or medium that generateslittle deterioration of the recording film due to repeated writing. Theinformation of the recording condition or medium can be prerecorded inthe medium as an identifier. Alternatively, the variation range can bealtered in accordance with the modulation method of the write data orthe recording condition such as overwriting frequency of the sector,whether the sector is directory area or not, or whether the sector is onthe guide groom or on the land (between the guide grooves).

[0022] A second method according to the present invention comprises thefollowing steps for recording and reproducing. The writing stepscomprise selecting one of two or more different methods for convertingwrite data into converted data, writing an identifier for identifyingthe method used as the converting method, together with the converteddata into an optical recording medium. The reproducing steps comprisereading the converted data and the identifier from the optical recordingmedium, and selecting one of two or more methods for restoring theoriginal data from the read data in accordance with the identifier.According to this method, the write data signals have different patternsby changing an order of blocks even if the same write data are writteninto the same part of the optical recording medium repeatedly. As aresult, a damage at a specific part of the recording film can bedispersed, and a deterioration of the recording film due to repeatedoverwriting can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In the accompanying drawings:

[0024]FIG. 1 is a block diagram of a writing and reading apparatusaccording to a first embodiment of the present invention;

[0025]FIG. 2 is a flow chart showing a process for overwriting a sectorof the optical disk in the apparatus shown in FIG. 1;

[0026]FIG. 3 is a block diagram of a system controller circuit of theapparatus shown in FIG. 1;

[0027]FIG. 4 shows an example of a delay time controller circuit of theapparatus shown in FIG. 1;

[0028]FIG. 5 shows another example of a delay time controller circuit ofthe apparatus shown in FIG. 1;

[0029]FIG. 6A is a graph showing the relationship between a variationrange of the start point for writing in a general area and an errorrate;

[0030]FIG. 6B is a graph showing the relationship between a variationrange of the start point for writing in a directory area and an errorrate;

[0031]FIG. 7 shows a record format when the variation range of the startpoint for writing was set at 16T in the apparatus shown in FIG. 1;

[0032]FIG. 8 shows a record format when the variation range of the startpoint for writing was set at 160T in the apparatus shown in FIG. 1;

[0033]FIG. 9 is a block diagram of a variation of the writing andreading apparatus shown in FIG. 1;

[0034]FIG. 10 is a flow chart showing a process for overwriting a sectorof the optical disk in the apparatus shown in FIG. 9;

[0035]FIG. 11 shows a record format when the variation range of thestart point for writing was set at 16T in the apparatus shown in FIG. 9;

[0036]FIG. 12 shows a record format when the variation range of thestart point for writing was set at 160T in the apparatus shown in FIG.9;

[0037]FIG. 13 is a block diagram of another variation of the writing andreading apparatus shown in FIG. 1;

[0038]FIG. 14 is a block diagram of another variation of the writing andreading apparatus shown in FIG. 1;

[0039]FIG. 15 is a block diagram of a writing and reading apparatusaccording to a second embodiment of the present invention;

[0040]FIG. 16 is a flow chart showing a process for overwriting a sectorof the optical disk in the apparatus shown in FIG. 15;

[0041]FIG. 17 is a flow chart showing a process for reproducing datawritten in a sector in the apparatus shown in FIG. 15;

[0042]FIG. 18 shows write data before permutation and the data afterpermutation in the apparatus shown in FIG. 15;

[0043]FIG. 19 shows read data before restoring and the data afterrestoring in the apparatus shown in FIG. 15;

[0044]FIG. 20 is a block diagram of a permutation method decisioncircuit and a permutation circuit of the apparatus shown in FIG. 15;

[0045]FIG. 21 is a block diagram of a permutation data detector circuitand a restoring circuit of the apparatus shown in FIG. 15;

[0046]FIG. 22 is a block diagram of a system controller circuit of theapparatus shown in FIG. 15;

[0047]FIG. 23 is a block diagram of a variation of the writing andreading apparatus shown in FIG. 15;

[0048]FIG. 24 shows an example of interleaving and deinterleavingoperations in the apparatus shown in FIG. 23;

[0049]FIG. 25 is a block diagram of another variation of the writing andreading apparatus shown in FIG. 15;

[0050]FIG. 26 shows an example of bit shift and reverse bit shiftoperations in the apparatus shown in FIG. 25;

[0051]FIG. 27 is a cross section of an optical disk using a phase-changetype recording film in the prior art;

[0052]FIG. 28 is a block diagram of a writing and reading apparatus inthe prior art;

[0053]FIG. 29 shows write data, modulated laser power, record mark andrecord format in the prior art;

[0054]FIG. 30 is a block diagram of a system controller circuit if theapparatus shown in FIG. 28;

[0055]FIG. 31 is a block diagram of a variation of the apparatus shownin FIG. 1;

[0056]FIG. 32 is a flow chart showing a process for overwriting a sectorof the optical disk in the apparatus shown in FIG. 31;

[0057]FIG. 33 shows a record format when the variation range of thestart point for writing was set at 160T in the apparatus shown in FIG.31;

[0058]FIG. 34 shows an example of a first delay time controller circuitof the apparatus shown in FIG. 31; and

[0059]FIG. 35 shows an example of a second delay time controller circuitof the apparatus shown in FIG. 31.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0060] The writing and reading method and apparatus according to thepresent invention are further explained in detail using the figures andpreferred embodiments.

[0061] (First Embodiment)

[0062]FIG. 1 shows a block diagram of an apparatus for writing on andreading an optical recording disk according to a first embodiment of thepresent invention. FIG. 2 shows a flow chart for overwriting a sector ofthe optical disk in the apparatus shown in FIG. 1. A system controllercircuit 101 that is connected to a host computer detects an address dataof a sector to be overwritten in the optical disk 113 (Step 201 in FIG.2). Then the system controller circuit 101 outputs write data 102 (Step202).

[0063] An error correction and interleaving circuit 103 adds errorcorrection data to the write data, and performs an interleaving process(Step 203). The interleaved data is then modulated by a modulatorcircuit 104 (Step 204). These operations are the same as the prior artshown in FIG. 28.

[0064] The modulator circuit 104 outputs the modulated signal 105, whichis supplied to a delay time controller circuit 106. The delay timecontroller circuit 106 judges whether the area to be overwritten has ahigh frequency of overwriting, such as the directory area or not (Step205), and sets the variation range of the start point for writing to belarge in an area like the directory area (Step 206), or small in ageneral area (Step 207). The above-mentioned judgment is performedaccording to the identifier detected by an identifier detector circuit119. This identifier is prewritten in the ID area 601 (FIG. 29) or otherarea of each sector. The identifier detector circuit 119 detects theidentifier according to a predetermined timing generated by the systemcontroller circuit 101. The timing corresponds to the position of thewritten identifier in the optical disk 113, and outputs the detectedresult signal.

[0065] The delay time controller circuit 106 delays the modulated data105 at random within a predetermined delay time corresponding to thevariation range set for the modulated data 105 (Step 208). The delaytime controller circuit will be explained in detail later.

[0066] A synthesizer circuit 109 adds a synchronizing signal (VFO)generated by a synchronizing signal generator circuit 107 and dummy datagenerated by a dummy data generator circuit 108 to each data block to bewritten into a sector, so as to generate a write data signal 118 (Step209). The write data signal 118 is supplied to a laser driver circuit110, which generates a laser driving signal 111 to drive a laser housedin an optical head 112. After the intensity of the laser beam ismodulated (Step 210), the laser beam irradiates the optical disk 133 forwriting data into the sector.

[0067] The system controller circuit has a configuration as shown inFIG. 3. This configuration differs from that of the prior art shown inFIG. 30 in that an identification timing generator circuit 2701 suppliesan identification timing signal to the identifier detector circuit 119according to an address data and an address detection signal from anaddress data detector circuit 2603. The address data detecting signal isalso supplied to the delay time controller circuit 106.

[0068]FIG. 4 shows an example of the delay time controller circuit 106.Generally, a delay time controller circuit includes a write controlsection having two different write timings, and a selecting section forselecting one of the two write timings in accordance with theidentifier. In FIG. 4, the selecting section is a switching circuit 305.The write control section includes plural delay circuits 301, two clockgenerators 302, 303 that generate clock signals for the delay circuits301, and selector 304 that selects one of the delay circuits 301 forinputting the modulated data signal 105. Each delay circuit 301 includesshift registers, delay lines or counters.

[0069] The delay time controller circuit shown in FIG. 4 has two clockgenerator circuits 302, 303. The period of the clock generated by thefirst clock generator circuit 302 is T, and that generated by the secondclock generator circuit 303 is 10T.

[0070] If the switching circuit 305 selects the first clock generatorcircuit 302, the delay times of the delay circuits are 0, T, 2T, 3T, . .. , 16T respectively (First write timing). On the contrary, if thesecond clock generator circuit 303 is selected, the delay times of thedelay circuits 301 are 0, 10T, 20T, 30T, . . . 160T respectively (Secondwrite timing).

[0071] The actual operation of the delay time controller circuit 106shown in FIG. 4 is as follows.

[0072] When writing on the general area of the optical disk 113, theswitching circuit 305 selects the first clock generator circuit 302according to the signal 121 from the identifier detector circuit 119.The delay circuits 301 generate corresponding delay times 0-16T based onthe period T of the first clock. The address detection signal 120 makesthe selector 304 select one of delay circuits 301 at random. Theselected delay circuit is maintained until the next address is detected.

[0073] When writing on the directory area of the optical disk 113, theswitching circuit 305 selects the second clock generator circuit 303according to the signal 121 from the identifier detector circuit 119.The delay circuits 301 generate corresponding delay times 0-160T basedon the period 10T of the second clock. The address detection signal 120makes the selector 304 select one of delay circuits 301 at random. Thus,the variation range of the start point for writing data can be alteredbetween the directory area and other area by changing the clock (periodstep) for the plural delay times.

[0074]FIG. 5 shows another example of the delay time controller circuit106. In this circuit, the selecting section is a switching circuit 405.The write control section includes plural delay circuits 401, a clockgenerator circuit 402 that generates a clock signal for the delaycircuits 401, and two selectors 403, 404 that select one of the delaycircuits 401 for inputting the modulated data signal 105 through theswitching circuit 405.

[0075] The delay times of the delay circuits 401 are 0, T, 2T, 3T, . . .160T based on the clock period T. In other words, the step width is T,and the total width is 160T.

[0076] The delay time controller circuit shown in FIG. 5 has twoselectors 403, 404. If the switching circuit 405 selects the firstselector 403, the write timing is altered at random within the delaytime of 0-16T (First write timing). If the switching circuit 405 selectsthe second selector 404, the write timing is altered at random withinthe delay time of 0-160T (Second write timing).

[0077] The actual operation of the delay time controller circuit 106shown in FIG. 5 is as follows.

[0078] When writing on the general area of the optical disk 113, theswitching circuit 405 selects the first selector 403 according to thesignal 121 from the identifier detector circuit 119. The delay time isselected at random from 16 steps 0-16T. The selected delay time ismaintained until the next address is detected.

[0079] When writing on the directory area of the optical disk 113, theswitching circuit 405 selects the second selector 404 according to thesignal 121 from the overwrite frequency identifier detector circuit 119.The delay time is selected at random from 160 steps 0-160T. The selecteddelay time is maintained till detection of the next address. Thus, thevariation range of the start point for writing can be changed betweenthe directory area and other areas by changing the step number of theplural delay times.

[0080] The following explanation is about an example for confirming theeffect of this embodiment. The substrate of the optical disk 113 wasmade of a polycarbonate plate having a diameter of 130 mm and athickness of 0.6 mm. Pits are preformed on the substrate as addressdata, and guide grooves on which data are to be written are formed insector areas. A pitch of the guide grooves was 1.6 micron. Four layers,that is a protective film, a photosensitive film, a protective film anda reflection film were formed on the substrate by sputtering. Then, aprotective sheet was glued on the surface of the layers. The protectivefilm was made of ZnS—SiO₂, the photosensitive film was made of Te—Sb—Ge,and the reflection film was made of Al.

[0081] The above-mentioned optical disk was rotated at a linear speed of5 m/s by a spindle motor 113. A laser beam having a wavelength of 680 nmwas used for writing after being focused by an objective lens with anumerical aperture (N. A.) of 0.6.

[0082] Laser powers for writing and reading were set at Pp=10 mW, Pb=4mW, and Pr=1 mW. A method of (8-16) pulse width modulation was used formodulating the write data. The shortest mark length was 0.6 micron. Thedelay time controller circuit 106 was used, which selects the variationrange of the start point for writing by selecting the clock generatorcircuit 302 or 303 as shown in FIG. 1.

[0083] Under the above-mentioned conditions, the relationship betweenthe variation range of the start point for writing and an error rate ofread data is measured in each area. In the directory area, assumingsimilar data are to be written repeatedly in a real application, twopatterns of data were written repeatedly and the variation range was setwithin 0-160T (0-10T per a step). In the general area, thirty patternsof data were written repeatedly and the variation range was set within0-64T (0-4T per a step). The error rate of the read data was measuredafter overwriting 100,000 times.

[0084]FIG. 6A shows the relationship between the variation range of thestart point for writing and the error rate after overwriting 100,000times in the general area. FIG. 6B shows the relationship between thevariation range of the start point for writing and the error rate afteroverwriting 100,000 times in the directory area.

[0085] As understood from FIG. 6A and 6B, a better error rate afteroverwriting 100,000 times is obtained if the variation range of thestart point for writing becomes larger. It was also understood that theminimum variation range of the start point for writing in which the gooderror rate was obtained varied depending on the type of area (this meansa randomness of the write data).

[0086] In accordance with the above-mentioned result, the variationrange of the start point for writing was set as follows. To obtain theerror rate below 0.0005, the variation range of the start point forwriting was set at 16T (1T per a step) in the general area, and at 160T(10T per a step) in the directory area (variable variation range case).For comparison, first and second fixed variation range cases were alsoperformed. In the first fixed variation range case, the variation rangewas set at 16T (1T per a step) in both the general and directory areas.In the second fixed variation range case, the variation range was set at160T (10T per a step) in both the general and directory areas.

[0087]FIG. 7 shows a record format when the variation range of the startpoint for writing was set at 16T. A clock for writing data was the sameas the clock for generating delay times. A data volume that can bewritten in a sector was 1000 bytes. Data lengths of the VFO area anddummy data area were both 15 bytes when the start point for writingdidn't change.

[0088] In FIG. 7, (a) shows a record format in the case where the startpoint for writing didn't change. A data area 604 was provided with VFO603 and dummy data 605 after being provided with a delay time by thedelay time controller circuit 106, and then supplied to the laser drivercircuit 110 for generating the laser driving signal 111.

[0089] In FIG. 7, (b) shows a record format in the case where the startpoint for writing was shifted backward by 16T (i.e., 1 byte). In thiscase, the laser-driving signal 111 was generated 16T later than the caseof (a). In the writing process with the variation range of 16T, thegeneration timing of the laser driving signal corresponding to the dataarea varied within 16T by means of the delay time controller circuit. Asa result, the data written position in the sector varied within 16T (1byte).

[0090]FIG. 8 shows a record format when the variation range of the startpoint for writing was set at 160T. In FIG. 8, (a) is a record format inthe case where the start point for writing didn't change, (b) is in thecase where the start point for writing was shifted forward by 80T (i.e.,5 byte), and (c) is in the case where the start point for writing wasshifted backward by 80T (i.e., 5 byte). In the case (b), thelaser-driving signal 111 corresponding to the data area was generated80T earlier than the case (a). In the case (c), the laser-driving signal111 corresponding to the data area was generated 80T later than the case(a).

[0091] In the writing process with the variation range of 160T, thegeneration timing of the laser-driving signal varied within 160T bymeans of the delay time controller circuit. As a result, the datawritten position in the sector varied within 160T (10 byte). The ID areain the optical disk predetermines the length of the sector. Therefore,if the gap area 802 and the buffer area 806 have fixed lengths, the VFOand dummy data areas can decrease in their lengths as the writtenposition varies more widely in the sector.

[0092] In the first fixed variation range case mentioned above, thevariation range was 1 byte in both the directory area and other areas.Therefore, the length of the VFO area or dummy data area varied within15-16 bytes or 14-15 bytes. In other words, the shortest length of theVFO area or the dummy data area was 14 or 15 bytes. Similarly, in thesecond fixed variation range case, the variation range was 10 bytes inboth the directory area and other areas. Therefore, the length of theVFO area or dummy data area varied within 10-20 bytes, and the shortestlength of the VFO area or the dummy data area was 10 bytes.

[0093] On the other hand, in the variable variation range case mentionedabove, in the directory area, the variation range of the start point forwriting was 10 bytes, so the lengths of the VFO area and the dummy dataarea varied within 10-20 bytes as shown in FIG. 8. The shortest lengthof the VFO area or the dummy data area was 10 bytes. In the generalarea, the variation range of the start point for writing was 1 byte, sothe length of the VFO area or the dummy data area varies within 15-16bytes (or 14-15 bytes). The shortest length of the VFO area or the dummydata area was 14 or 15 bytes.

[0094] Under the above-mentioned conditions, the following experimentwas performed. Two patterns of data were written into the directory arearepeatedly, and thirty patterns of data were written into the generalarea repeatedly. Error states were investigated after overwriting 50,000times and 100,000 times.

[0095] Table 1 shows the comparison of the error state among the firstfixed variation range case, the second fixed variation range case andthe variable variation range case. In this table, “synchro error” meansa synchronizing error state making the data reproduction impossible whena phase-locked loop (PLL) circuit becomes out of lock. Similarly, “readerror” means a state of improper error correction making the datareproduction impossible. TABLE 1 Variation range After 50,000 After100,000 First fixed v.r. case General area  16T OK OK Directory area160T read error read error Second fixed v.r. case General area 160T OKsynchro error Directory area 160T OK synchro error Variable v.r. caseGeneral area  16T OK OK Directory area 160T OK synchro error

[0096] As shown in Table 1, in the first fixed variation range case, thegeneral area could be overwritten 100,000 times with no error, but thedirectory area had a read error after 50,000 times of overwriting. Awaveform distortion was observed in the read signal of the directoryarea. It is estimated that the read error was generated because thevariation range of the start point was too small for the write data withsmall randomness, so a local deterioration of the recording film wasgenerated.

[0097] In the second fixed variation range case, both the general anddirectory areas could be overwritten 50,000 times with no error, but hada synchronizing error after 100,000 times of overwriting. In thewaveform of the read signal after 100,000 times of overwriting, at least5 bytes of the VFO were missing. It is estimated that the synchronizingerror was generated when the length of the VFO area became short sincethe variation range of the start point was large, and the deteriorationof the recording film at the start point in the sector caused an unlockof the PLL circuit since the deteriorated portion in the VFO area becamerelatively large.

[0098] On the other hand, in the variable variation range case, with thegeneral and directory areas could be overwritten 50,000 times with noerror, and the general area could be overwritten 100,000 times with noerror. The reason why the general area could be overwritten 100,000times with no error may be that the deteriorated portion in the VFO areabecame relatively small by decreasing the variation range of the startpoint for writing, and increasing the length of the VFO area in thegeneral area. In addition, since the variation range of the start pointfor writing was set large in the directory area, a local deteriorationof the recording film was hardly generated in the data area. Thus, thedirectory area could be overwritten 50,000 times without error.

[0099] As explained above, this embodiment of the present inventionprovides a better method for writing and reading an optical recordingmedium, in which the variation range of the start point for writing canbe changed in accordance with a write condition. Therefore, a localdeterioration of the recording film due to repeated overwriting can bereduced by enlarging the variation range in the directory area that hasa tendency to have its recording film deteriorated early due to repeatedoverwriting. Thus, the number of times of overwriting can be increased.In addition, in the general area where the deterioration of therecording film is little, the number of times of overwriting in thegeneral area can be further increased by lengthening the VFO and/ordummy data area for relieving a deterioration at the start and endpoints in the sector.

[0100] In FIG. 1, the delay time controller circuit 106 is locatedbefore the synthesizer circuit 109. Alternatively, the delay timecontroller circuit 106 may be located after the synthesizer circuit 109as shown in FIG. 9. In this case, FIG. 10 shows a flow chart foroverwriting a sector of the optical disk. This flow chart differs fromthat shown in FIG. 2 in that the write data is delayed (Step 909) by thedelay time controller circuit 106 after being provided with the VFO 603and dummy data 605 (Step 905).

[0101]FIG. 11 shows a record format when the variation range of thestart point for writing was set at 16T. In FIG. 11, (a) shows a recordformat in the case where the start point for writing didn't alter, and(b) shows a record format in the case where the start point for writingwas delayed 16T (i.e., 1 byte).

[0102]FIG. 12 shows a record format when the variation range of thestart point for writing was set at 160T. In FIG. 12, (a) is a recordformat in the case where the start point for writing didn't alter, (b)is in the case where the start point for writing was shifted forward by80T, and (c) is in the case where the start point for writing wasdelayed 80T.

[0103] These cases differ from the cases shown in FIGS. 7 and 8 in thatall the start points for writing the VFO 603, the data area 604 and thedummy data 605 were varied. As a result, an influence of thedeterioration at the start and/or end point of the sector is reduced,since the lengths of the VFO and dummy data areas were not shortened.

[0104] A block diagram of a writing and reading apparatus as a variationof this embodiment is shown in FIG. 31. This apparatus includes firstand second delay time controller circuits 3101, 3102 before and afterthe synthesizer circuit 109. The first delay time controller circuit3101 delays the modulated data signal within the variation range of0-144T. The second delay time controller circuit 3102 delays the writedata signal within the variation range of 0-16T. FIG. 32 shows a flowchart for overwriting a sector of the optical disk in the apparatusshown in FIG. 31. This flow chart shown in FIG. 32 differs from that ofFIG. 2 in the following steps. The first delay time controller circuit3101 delays the modulated data signal within the variation range of0-144T only when the area to be written has a high frequency ofoverwriting (Step 3206). After the VFO 603 and the dummy data 605 areadded to the write data (Step 3207), the second delay time controllercircuit 3102 delays every data signal not depending on the frequency ofoverwriting, within the variation range of 0-16T (Step 3208).

[0105]FIG. 34 shows an example of the first delay time controllercircuit 3101 of the apparatus shown in FIG. 31. In this block diagram,the selecting section is a switching circuit 3406. The write controlsection 3405 includes plural delay circuits 3401, a clock generatorcircuit 3402 that generate clock signals for the delay circuits 3401,and selector 3403 that selects one of the delay circuits 3401 forinputting the modulated data signal 105.

[0106] The delay times of the delay circuits 3401 are set to 0, T, 2T, .. . , 144T respectively based on the clock period T. In other words, thestep width is T, and the total width is 144T.

[0107] If the switching circuit 3404 selects the selector 3403, thewrite timing is altered at random within the delay time of 0-144T by oneof delay circuits 3401 (First write timing). If the switching circuit3404 does not select the selector 3403, the write data is outputteddirectly without passing through any delay circuit. In this case, thedelay time is zero, and the write timing is constant (Second writetiming).

[0108] The actual operation of the delay time controller circuit shownin FIG. 34 is as follows. When writing on the directory area of theoptical disk 113, the switching circuit 3404 selects the selector 3403according to the signal 121 from the identifier detector circuit 119.The delay time is selected at random from 144 steps 0-144T. The selecteddelay time is maintained until the next address is detected.

[0109] When writing on the general area of the optical disk 113, theswitching circuit 3404 selects the direct pass to the output withoutpassing through any delay circuit, so that the delay time is alwayszero.

[0110]FIG. 35 shows an example of the second delay time controllercircuit 3102 of the apparatus shown in FIG. 31. In this block diagram,the write control section 3504 includes plural delay circuits 3501, aclock generator circuit 3502 that generate clock signals for the delaycircuits 3501, and selector 3503 that selects one of the delay circuits3501 for inputting the modulated data signal 105.

[0111] The delay times of the delay circuits 3501 are set to 0, T, 2T, .. . , 16T respectively based on the clock period T. In other words, thestep width is T, and the total width is 16T.

[0112] In the actual operation, the delay time controller circuit shownin FIG. 35 works as follows. In every recording area of the opticaldisk, i.e., not depending on overwriting frequency, the selector 3503selects one of 16 delay times 0-16T at random in accordance with theaddress detection signal 120 given by the system controller circuit. Theselected delay time is maintained until the next address is detected.

[0113] By using two delay time controller circuits shown in FIGS. 34 and35, the number of steps of delay times can be changed, so that thevariation range of the start point for writing is changed between 160T(144T+16T) for the directory area and 16T for the general area.

[0114] Record formats for this embodiment of FIG. 31 are explainedbelow. When the variation range of the start point for writing is 16T,the record format is the same as shown in FIG. 11, in which (a) is inthe case where the start point for writing didn't alter, and (b) is inthe case where the start point for writing was delayed 16T.

[0115] When the variation range of the start point for writing is 160T,the record format is as shown in FIG. 33. In this figure, (a) shows arecord format in the case where the start point for writing was shiftedforward by 80T, and (b) shows a record format in the case where thestart point for writing was shifted backward (i.e., delayed) by 80T. Inthese cases, the start point for writing the VFO 603 and the dummy data605 varies within the variation range of one byte, and the start pointfor writing in the data area varies within the variation range of tenbytes.

[0116] The configuration shown in FIG. 9 is preferable if the spindlemotor 114 has a small jitter, or the laser driver circuit 110 has acapability of fast power control operation. On the contrary, if thejitter of the spindle motor 114 is not small, or the capability of thelaser driver circuit 110 is not high, it is preferable to adopt theconfiguration shown in FIG. 1, and to secure the constant gap area 602and buffer area 606 independently from the varying start point forwriting as shown in FIGS. 7 and 8. The configuration shown in FIG. 31ensures the gap area 602 and the buffer area 606 properly. Anyconfiguration shown in FIG. 1, 9 or 31 according to the presentinvention can enhance the number of times of overwriting.

[0117] Another method for high-density recording has been proposed,where lands between guide grooves are also used for recording. In thiscase, a thermal stress generated at the periphery of the record mark isdifferent between the guide groove and the land because their sectionshave different shapes at the periphery of the record mark. Therefore,the level of a deterioration generated after overwriting is differentbetween the guide groove and the land even if the overwriting isrepeated same times.

[0118] To improve the above-mentioned problem, it may be preferable toadopt the configuration of the writing and reading apparatus shown inFIG. 13. This configuration differs from that of FIG. 1 in that thedelay time controller circuit 106 set the variation range of the startpoint for writing in accordance with an identified result supplied froma land/guide groove identifier detector circuit 1201. In this case, thevariation range of the start point for writing in the land is setdifferent from that for writing in the guide groove, so that the numberof times of overwriting is enhanced.

[0119] A pulse width modulation method in which both edges of the recordmark have information is also proposed for a high-density recording.However, in this pulse width modulation method the recording film tendto wear earlier than in the pulse position modulation method in whichthe position of the record mark provides information, because the formerusually makes longer marks than the latter. Moreover, the pulse widthmodulation cannot reproduce data correctly if the edge of the recordmark is not detected precisely. Therefore, reproduction capability ofthe pulse width modulation is affected very much by the deterioration ofthe recording film. Thus, in the pulse width modulation it is moredifficult to reproduce data correctly than in the pulse positionmodulation under the same deterioration level of the recording film.

[0120] Considering such a problem, it is also preferable to adopt aconfiguration of the writing and reading apparatus as shown in FIG. 14.This configuration differs from that shown in FIG. 1 in that the delaytime controller circuit 106 sets the variation range of the start pointfor writing in accordance with a detection result by a modulation methodidentifier detector circuit 1301. In this case, the variation range ofthe start point for writing with the pulse width modulation is setdifferent from that for writing with the pulse position modulation, sothat the number of times of overwriting is enhanced.

[0121] The variation range of the start point for writing, the changestep number, the change interval, or the record format mentioned aboveis an example, and the proper values should be selected for themaccording to the recording condition or medium. In addition, thevariation range of the start point for writing can be changed amongthree or more values by combining the modulation method, the overwritefrequency, land/guide and other elements.

[0122] (Second embodiment)

[0123]FIG. 15 shows a block diagram of an apparatus for writing on andrecording an optical recording disk according to a second embodiment ofthe present invention. FIG. 16 shows a flow chart for overwriting asector of the optical disk in the apparatus shown in FIG. 15.

[0124] This embodiment differs from the prior art in the followingprocess. A permutation method decision circuit 1401 decides thepermutation method of the write data 102 at random (Step 1503). Apermutation circuit 1402 divides the write data into plural groups andchanges the order of the groups according to an instruction from thepermutation method decision circuit 1401 to obtain converted data 1405(Step 1504). The permutation circuit 1402 also adds the permutation dataas an identifier for restoring original data from the converted data(Step 1505).

[0125]FIG. 17 is a flow chart showing a process of reproducing a datawritten in a sector. This process differs from that of the prior art inthe following steps. After error correction and deinterleaving (Step1604), the permutation data detector circuit 1403 detects thepermutation data that is the identifier for restoring the original data(Step 1605). A restoring circuit 1404 restores the original data basedon the identifier (Step 1606).

[0126] An example of the permutation and restoring of data is explainedbelow using FIGS. 18 and 19.

[0127] The division and permutation of write data are performed asfollows. Dividing positions are determined at random concerning a seriesof write data shown in (a) of FIG. 18. Then the series of write data isdivided, changed in order, and provided with permutation data as anidentifier showing the divided position, so as to make a series of datashown in (b) of FIG. 18. For example, if the write data are divided atthe 20th byte and changed in order (this means a conversion method), anidentifier showing the 20th byte is added to the converted data. It isnot required to write the identifier into every sector. The identifiermay be written into the directory (directory) area. The conversionmethod is not required to be changed for each sector, but can be samefor plural sectors in a serial writing. Then, the error correction dataare added and the interleave process is performed.

[0128] As shown in (a) of FIG. 19, when reproducing data, after errorcorrection and deinterleaving, the identifier, i.e., permutation data,added to the tail of the converted data, are detected. For example, ifthe permutation data show the 20th byte, the last 20 bytes of theconverted data are divided and added to the head of the data (this meansa restoring method), so that the original data are obtained as shown in(b) of FIG. 19

[0129] The dividing position is determined at random by the permutationcircuit 1402 at every writing of a sector, so a different modulated datasignal is used for each writing even if the same data are written intothe same sector repeatedly. As a result, the optical disk 113 is writtenwith different write data signals except the VFO and RESYNC areas.Therefore, a probability of forming a record mark 2501 on the guidegroove 2303 is substantially uniform in a sector of the optical disk113. Thus, local damage of the recording film due to repeatedoverwriting is relieved.

[0130]FIG. 20 shows an example of the permutation method decisioncircuit 1401 and the permutation circuit 1402. In this figure, a randomnumber generator 2901 generates random numbers, when being triggered bya permutation-timing signal. If each sector of the optical disk isaccessed at random, a usual counter circuit can be used instead of therandom number generator to obtain the same effect. An address presetcircuit 2902 gives an initial memory address to a memory 2903 forreading the memory 2903. In the above-mentioned example, the initialmemory address is an address corresponding to 20th byte from the head ofthe write data stored in the memory 2903. The memory 2903 stores thewrite data and outputs the stored data from the given initial memoryaddress to the tail address and the remaining part of the data from thehead address in order. Thus, the memory 2903 outputs the converted data.A synthesizer circuit 2904 add an identifier indicating the initialmemory address to the data.

[0131]FIG. 21 shows an example of the permutation data detector circuit1403 and the restoring circuit 1404. A detector circuit 3001 detects theidentifier from the decoded data after error correction anddeinterleaving. A hold circuit 3002 holds the identifier and outputs thesame to an address preset circuit 3003 until the next identifier isdetected. The address preset circuit 3003 gives an initial memoryaddress to a memory 3004. In the example mentioned above, the initialmemory address is an address corresponding to 20th byte from the tail ofthe converted data stored in the memory 3004. The memory 3004 stores theconverted data and outputs the stored data from the given initial memoryaddress to the tail address and the remaining part of the data from thehead address in order. Thus, the memory 3004 outputs the restored data.

[0132]FIG. 22 shows the system controller circuit of this embodiment.This circuit differs from that of the prior art in that a permutationtiming generator circuit 2801 outputs a permutation timing signal to thepermutation method decision circuit 1401 in accordance with the addressdata and the address data detecting signal from the address datadetector circuit 2603.

[0133] The following explanation is about an example for confirming theeffect of this embodiment. The substrate of the optical disk 113 wasmade of a polycarbonate plate having a diameter of 130 mm. Pits arepreformed on the substrate as address data, and guide grooves forwriting are formed in sector areas. Four layers, that is a protectivefilm, a photosensitive film, a protective film and a reflection filmwere formed on the substrate by sputtering. Then, a protective sheet wasglued on the surface of the layers.

[0134] The protective film was made of ZnS—SiO₂, the photosensitive filmwas made of Te—Sb—Ge, and the reflection film was made of Al. Thisoptical disk was rotated at a linear speed of 5 m/s by the spindle motor113. A laser beam having a wavelength of 680 nm was used for writing,after being focused by an objective lens with a numerical aperture (N.A.) of 0.6. Laser powers for writing and reading were set at Pp=10 mW,Pb=4 mW, and Pr=1 mW. A method of (8-16) pulse width modulation was usedfor modulating the write data. The shortest mark length was 0.6 micron.

[0135] According to the above-mentioned condition, the write data wereoverwritten 100,000 times into the same sector. The error rate ofdemodulated data was measured for each overwriting with two methods forcomparison. The first method used the permutation by dividing the writedata and changing the order according to the present invention. Thesecond method didn't use the permutation as the prior art. In bothmethods, the write data for a sector includes 500 bytes. In the firstmethod using the permutation, the dividing position was selected atrandom by a step of one byte. The measured result is shown in Table 2.TABLE 2 Error rate after 100,000 times overwrite Without permutation0.011 With permutation 0.000002

[0136] As shown in Table 2, the first method with the permutation hassmaller error rate than the second method without permutation, so thatthe number of times of overwriting is enhanced.

[0137] As mentioned above, according to the writing and reading methodof the optical recording medium according to this embodiment of thepresent invention, the write data signals have different patterns byconverting into plural patterns even if the same write data are writteninto the same sector of the optical recording medium repeatedly. As aresult, a damage at a specific part of the recording film can bedispersed, and a deterioration of the recording film due to repeatedoverwriting can be reduced.

[0138] The number of divided blocks and other parameter in thisembodiment is an example, and should be selected adequately inaccordance with the record condition or medium.

[0139] The method for dividing a series of write data should not belimited to the example mentioned in this embodiment, but may be anymethod that can convert write data into plural different converted data.

[0140] For example, as shown in FIG. 23, a different interleaving methodcan be used by adding another pair of interleaving circuit anddeinterleaving circuit to the configuration of FIG. 15. In theconfiguration shown in FIG. 23, one of two error correction andinterleaving circuits 103, 1903 is selected by a first selecting circuit1902 at random controlled by an interleaving method decision circuit1901, and an identifier indicating the selected method is added to theconverted data in the recording process. In the reproducing process, oneof two error correction and deinterleaving circuits 117, 1904 isselected by a second selecting circuit 1905 according to the identifierdetected by an interleaving method detector circuit 1906. Theinterleaving method decision circuit 1901 includes a random numbergenerator circuit or a counter circuit in the same way as thepermutation method decision circuit shown in FIG. 15.

[0141]FIG. 24 shows an example of interleaving and deinterleavingoperations. In FIG. 24, (a) shows write data before interleaving, (b)shows the data after interleaving and provided with an identifierindicating the interleaving method, (c) shows read data beforedeinterleaving, and (d) shows data after deinterleaving in accordancewith the detected identifier. In this case, the same process as theinterleaving can perform division and permutation of the write data, sothat the configuration of the writing and reading apparatus can besimplified.

[0142] The method for converting a series of write data into one of twoor more different series of converted data may be the following method.

[0143]FIG. 25 shows a configuration for generating a series of converteddata from a series of write data by a scrambling method, which comprisesa bit shift process of the write data. In the recording process, a bitshift method decision circuit 2101 decides a shift bit number at random,and a bit shift circuit 2102 performs the bit shift process by a unit ofone or more bits according to the decided shift bit number to obtainconverted data 1405. An identifier indicating the shift bit number isadded to the converted data. These steps correspond to the convertingmethod. In the reproducing process, a bit shift identifier detectorcircuit 2103 detects the identifier, and a reverse bit shift circuit2104 performs the reverse bit shift process by a unit of one or morebits according to the detected identifier. These steps correspond to therestoring method. The bit shift method decision circuit 2101 includes arandom number generator circuit or a counter circuit in the same way asthe permutation method decision circuit shown in FIG. 15.

[0144]FIG. 26 shows an example of the bit shift and reverse bit shiftoperations. In FIG. 26, (a) shows write data before bit shift, (b) showsthe data after bit shift and provided with an identifier indicating thebit shift method, (c) shows read data before reverse bit shift, and (d)shows data after reverse bit shift in accordance with the detectedidentifier. In this case, a large memory for permutation is notrequired, so that the configuration of the writing and reading apparatuscan be simplified.

[0145] In the second embodiment, by changing the start point for writinga sector with a modulated data signal at random, the RESYNC areaincluded in the data area can also be written in a different position,so the number of times of overwriting is further enhanced.

[0146] The invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Theembodiments disclosed in this application are to be considered in allrespects as illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims rather than by the foregoingdescription, all changes that come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

1. A method for writing on and reading an overwritable optical recordingmedium having sector format, the method comprising the steps of:modulating write data to make a modulated data signal corresponding to arecord pattern of the optical recording medium; selecting a first orsecond record timing; changing a start point for writing the modulateddata signal into the sector at random within a first variation rangewhen the first record timing is selected; and changing a start point forwriting the modulated data signal into the sector at random within asecond variation range that is larger than the first variation rangewhen the second record timing is selected.
 2. The method according toclaim 1, wherein the selecting step is performed based on an identifierwritten in the optical recording medium.
 3. The method according toclaim 1, wherein the selecting step is performed based on the modulationmethod used for modulating the write data.
 4. The method according toclaim 1, wherein the selecting step is performed so that the firstrecord timing is selected when overwrite frequency is low, and thesecond record timing is selected when the overwrite frequency is high.5. The method according to claim 1, wherein the selecting step isperformed so that the second record timing is selected when a directoryarea is overwritten, and the first record timing is selected whenanother area is overwritten.
 6. The method according to claim 1, whereinthe selecting step is performed based on the detected result of whetherthe sector to be written is on a guide groove or on a land between guidegrooves.
 7. The method according to claim 3, wherein the selecting stepis performed so that the first record timing is selected when themodulated data signal is generated by a pulse position modulation, andthe second record timing is selected when the modulated data signal isgenerated by a pulse width modulation.
 8. The method according to claim1, wherein the start point for writing the modulated data signal intothe sector is changed at random by fixing a start point for writing VFOto be added to the head of the modulated data signal, and by changingthe length of the VFO.
 9. The method according to claim 1, furthercomprising a step of adding dummy data to the tail of the modulated datasignal and changing the length of the dummy data in accordance with thevarying start point of the modulated data signal so that the end pointof the dummy data is fixed in the sector.
 10. An apparatus for writingon and reading an overwritable optical recording medium having sectorformat, the apparatus comprising: a modulator circuit for modulatingdata to make a modulated data signal corresponding to a record patternof the optical recording medium; a first delay circuit for delaying thestart point for writing the modulated data signal given by the modulatorcircuit in the sector at random within a first variation range; a seconddelay circuit for delaying the start point for writing the modulateddata signal given by the modulator circuit in the sector at randomwithin a second variation range that is larger than the first variationrange; and a switching circuit for selecting the first or second delaycircuit.
 11. The apparatus according to claim 10, wherein the first andsecond delay circuits select one of plural steps of delay time at randomand a step height of the second delay circuit is larger than that of thefirst delay circuit.
 12. The apparatus according to claim 10, whereinthe first and second delay circuits select one of plural steps of delaytime at random and a step number of the second delay circuit is largerthan that of the first delay circuit.
 13. A method for writing andreading an overwritable optical recording medium, the method comprisingthe steps of: selecting one of plural different conversion methods forconverting write data into converted data; writing the converted dataand an identifier indicating the selected conversion method into theoptical recording medium; reading the converted data and the identifierwritten in the optical recording medium; selecting one of pluraldifferent restoring methods according to the read identifier; andrestoring the original data from the read data by using the selectedrestoring method.
 14. The method according to claim 13, wherein the stepof selecting the conversion method is performed at every overwriting.15. The method according to claim 14, wherein the plural differentconversion methods for converting write data are interleaving methods.16. The method according to claim 14, wherein the plural differentconversion methods for converting write data are scrambling methods. 17.The method according to claim 13, wherein the start point for writingthe modulated data signal in a sector is changed at random.
 18. Anapparatus for writing on and reading an overwritable optical recordingmedium, the apparatus comprising: a first switching circuit forselecting one of plural different conversion methods; a convertercircuit for converting write data into converted data according to theselected conversion method; a writing circuit for writing the converteddata and an identifier indicating the selected conversion method intothe optical recording medium; a reproducing circuit for reproducing theconverted data and the identifier written in the optical recordingmedium; a detector circuit for detecting the identifier; a secondswitching circuit for selecting one of plural different restoringmethods based on the detected identifier; and a restoring circuit forrestoring an original data from the converted data according to theselected restoring method.